In a wireless communication system, inter-symbol interference (ISI) between received signals is usually caused by a multi-path fading effect in a radio channel. To remove the ISI, a receiver is provided with an equalizer that needs information of channel impulse response (CIR) to operate, and therefore estimation of the CIR plays a critical part in a mobile radio system.
The OFDM, an important communication technology in the wireless communication field, is capable of increasing a data transmission rate. For example, the OFDM technology is implemented in IEEE 802.11a, which provides a data transmission rate up to 54 Mbps, whereas a data transmission rate is only 11 Mbps in IEEE 802.11b without the OFDM technology. To effectively estimate CIR of an OFDM system to remove ISI between symbols and thus to fully benefit from a high transmission rate of the OFDM system is obviously an important subject. In the OFDM system, estimation of preliminary estimated frequency-domain channel responses H(k) is commonly achieved by a least square difference calculation on a frequency-domain transmitting value and a frequency-domain receiving value of a pilot symbol at a position of each of pilot sub-carriers. A relationship between the frequency-domain transmitting value and the frequency-domain receiving value is represented by Y(k)=H(k)X(k)+Nk, where Y(k) represents a signal received by a receiver, X(k) represents a signal received by a transmitter, H(k) represents a frequency-domain channel response, and Nk represents noises. In an OFDM channel, X(k) transmitted via pilot sub-carriers is known, and X(k) transmitted via data sub-carriers is unknown. Accordingly, H(k) corresponding to a pilot symbol is first obtained from
      H    ⁡          (      k      )        =            Y      ⁡              (        k        )                    X      ⁡              (        k        )            (i.e., the noises Nk are omitted), and frequency-domain channel impulse responses H(k) corresponding to other data sub-carriers are interpolated according to channel estimation. Therefore, when the frequency-domain channel impulse response H(k) is obtained, X(k) transmitted via the data sub-carriers is calculated as
      X    ⁡          (      k      )        =                    Y        ⁡                  (          k          )                            H        ⁡                  (          k          )                      .  
The preliminary estimated frequency-domain channel responses H(k) only comprise calculated values at positions where the frequency k corresponds to pilot sub-carriers, and frequency-domain channel response values corresponding to other data sub-carriers are first defined as 0. That is, in practice, an Inverse Fast Fourier Transform (IFFT) in an OFDM channel estimating apparatus only generates non-zero values at positions of the pilot sub-carriers, and thus the scale of the IFFT calculation may have room for improvement.
Therefore, a channel estimating apparatus and a method thereof are needed to properly adjust sampling points of an IFFT, reduce the scale of the IFFT and maintain a same channel estimating efficiency, thereby reducing circuit costs.